This invention belongs to the field of pressure sensors and more particularly sensors designed for monitoring the intracorporeal pressure of a patient during a period of medical surveillance, in particular during a surgical intervention.
Its object is a self-calibrating device for measuring intracorporeal pressure, combining a sensor with a piezoelectric transducer that reacts under the action of pressure and an actuator that makes it possible to produce, by specified amplitude polarization, a calibrated deformation at the level of the sensor.
The pressure measurements are commonly used to judge the state of vital physiological functions, or the gravity of a trauma, a pathology that produces abnormally high compression of an organ. These measurements of pressure, in particular arterial pressure and intracranial pressure, are taken daily in emergency services, in particular in intensive care.
The measurement of arterial pressure is necessary in numerous situations such as states of shock not responding in the first hour to medical management (septic shock, cardiogenic shock), refractory convulsive status epilepticus or acute respiratory distress syndrome. The post-operative management of multiple situations such as renal grafting, major orthopedic surgery, or major digestive surgery, where extended intervention requires many volume infusions that can lead to difficult post-operative hemodynamic management, also requires close monitoring of arterial pressure.
The measurement of the intracranial pressure is indicated in neuro-traumatology in cases of serious cranial traumas and also in certain cases of relational cerebral edemas, meningeal hemorrhages or aggressively growing cerebral tumors. All of these pathological processes have in common the development, by the action of mass, of intracranial hypertension, since the cranium cannot be expanded in an adult. An increase in the intracranial pressure produces a reduction of the cerebral perfusion pressure that may be responsible for a decrease in the cerebral blood flow and for creating an energy threat situation that can lead to ischemia, i.e., the death of the cerebral tissue.
It is understood that the monitoring and regulation of the intracorporeal pressure can assume an often vital importance of the first order in contexts that are particularly difficult for the practitioners. Means for measuring both the intracranial pressure as well as the arterial pressure have therefore been sought for a long time—means that are suitable during work in an operating room or in a medical environment, requiring tools that are compact and easy to handle and that can provide reliable information continuously and under strict sanitization conditions.
In a conventional way, the measurement of arterial pressure is taken using a sensor that is inserted into the radial or femoral artery by means of a catheter. The principle of the sensor is that of a membrane that is coupled to gauges with resistors placed on a Wheatstone bridge. The resistance variations due to the compression or to the extension of gauges connected to the membrane are transformed into an electrical signal that is proportional to the arterial pressure. The resistors can be replaced by a system that induces a variation of an electromagnetic coupling based on movements of the membrane. The sensor can be external, which presents various drawbacks linked to the removal of the sensor, in particular in connection with effects of damping and deformation of the pressure signal and a limitation of the frequency response.
Integrated pressure sensors in the implanted end of a catheter are also used increasingly frequently, in particular in an operating room. Since 1980, sensors based on fiber-optic or piezoelectric transducer technologies (in particular with piezoresistive gauges) made of silicon have commonly been used. The gauge components are encapsulated at the end of a catheter made of nylon with an overall diameter that varies from 1.17 mm to 2.67 mm for the commercialized models. Owing to the miniaturization of the measuring probe, it has been possible to limit the risks of thrombosis during the taking of the arterial pressure, and the trauma linked to the intracranial implantation of the sensor has been minimized. Thus, in recent years, these devices have made it possible to measure the arterial pressure and the intracranial pressure at a lower cost, in a routine manner, with a much better precision of measurements, a reduced invasive nature and better controlled septic risks.
The fact remains that these techniques still exhibit drawbacks and call for innovations. A major problem with silicon-gauge sensors and especially with sensors with actual optical fibers is their tendency to drift, regardless of conditions of storage and use. Values of 2 to 5 mm of Hg per 24-hour cycles are commonly observed. It is therefore essential to carry out a re-calibration at regular intervals. At present, however, the calibration is carried out only in an extracorporeal manner, which very seriously increases the risks of infection for the patients.
Actually, the calibration of the pressure sensors is in general carried out by applying a known pressure on the membrane of the sensor and by verifying that the electrical quantity delivered by the latter is equivalent to the stress exerted. In practice, whereby the sensors that are used in the medical medium deliver a pressure value that is equal to the difference between the measured pressure and the atmospheric pressure, a sensor is calibrated at atmospheric pressure: if it is correctly calibrated, the measured pressure should be zero when it is held in the open air or immersed in a small volume of sterile water.
The handling of an instrument that is placed in the body, then withdrawn and inserted again is, however, delicate by nature and the source of infectious complications whose consequences may prove extremely serious. For the measurement of the intracranial pressure in particular, the emergence of the physical, electrical or optical link with the outside medium at the level of the scalp is always a critical point. It is a possible septic entryway to the intradural medium, which is completely vulnerable to infection. The periodically repeated installation of a piece of equipment therefore significantly multiplies the risks of infection. Thus, because of a high septic risk, it is preferable that a placed sensor no longer be withdrawn to undergo periodic re-calibrations.